In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating the fibrous material in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.
U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere commonly involves (1) a chain scission and oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, in the past the stabilization reaction must by necessity at least initially be conducted at relatively low temperatures (i.e. well below 300.degree. C.), since the cyclization reaction is known to be exothermic in nature and must be controlled if the original configuration of the material undergoing stabilization is to be preserved in the absence of undesirable fiber coalescence which impairs the tenacity values of the final product. Accordingly, the stabilization reaction has tended to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Representative processes proposed to shorten the time period required for the stabilization reaction include: U.S. Pat. Nos. 3,416,874; 3,539,295; 3,592,595; 3,647,770; 3,650,668; 3,656,882; 3,656,883; 3,708,326; 3,720,759; 3,729,549; 3,813,219; 3,814,577; 3,817,700; 3,820,951; 3,850,876; 3,923,950; 4,002,426; 4,004,053, and British Pat. Nos. 1,253,827; 1,260,449; 1,264,763; 1,375,136; and 1,448,468.
In U.S. Pat. Nos. 3,767,773 and 3,900,285; British Pat. No. 1,280,850; Belgian Pat. No. 743,458; Netherlands Pat. No. 6,919,555; German Pat. Nos. 2,054,255 and 2,358,853; U.S.S.R. Pat. No. 389,012; and Melliand Textilberichte, Vol. 38, Pages 65-68 by F. Schoutenden (1957) is disclosed the treatment of acrylic fibers with hydroxylamine. In U.S. Pat. No. 3,497,318; British Pat. No. 1,213,387; and the Indian Journal of Technology, Vol. 13, Pages 277-280 by Iqbal Singh (June, 1975) is disclosed the treatment of acrylic fibers with inter alia potassium permanganate.
Prior art processes which have attempted to shorten the time required for the thermal stabilization of acrylic fibers to render them suitable for carbon fiber formation commonly have resulted in at least some level of undesirable coalescence of adjoining fibers particularly if the multifilamentary material is contacted with heated rolls during the stabilization reaction. Points of coalescence in the fibers have been found to lead to diminished tenacity values in the desired fibrous carbonaceous product.
It is an object of the present invention to provide an improved process for the thermal stabilization of acrylic fibers.
It is an object of the present invention to provide an overall improved process for the thermal stabilization of acrylic fibers which can advantageously be carried out in a relatively brief period of time in the substantial absence of undesirable fiber coalescence.
It is another object of the present invention to provide an improved process for the thermal stabilization of acrylic fibers which readily can be carried out on a continuous basis.
It is a further object of the present invention to provide an improved process for the thermal stabilization of acrylic fibers wherein the fibers are capable of being heated in a gaseous oxygen-containing atmosphere while undergoing thermal stabilization which is at a more highly elevated temperature than would otherwise be possible thereby expediting the desired stabilization.
These and other objects, as well as the scope, nature, and utilization of the specifically claimed overall process will be apparent to those skilled in the art from the following description and appended claims.